High
Performance Glass--Continuously Improving the Energy Efficiency of Windows
by Frank Davey

Architects
continually look for new types of reflective and low-E coatings to achieve a
certain look as well as the best possible energy efficiency.

In
the last ten years, the residential and commercial window markets have undergone
a major technological revolution. Windows are no longer considered the weak link
in energy-efficient building design. New and improved coating technologies help
architects continue to expand glass applications without sacrificing energy
efficiency. Some innovations include: new reflective and low-E coatings,
low-conductance gas fills, warm-edge spacers, thermally improved sashes and
frames, solar control glazings and coatings and improved weatherstripping
products.

Current
Technology

For
commercial buildings, the most important consideration in choosing the proper
glazing is solar heat gain control. However, heat loss must also be evaluated,
particularly in cold climates. Utilizing passive solar heat gain efficiently to
reduce heating costs is an important consideration. This “free” heat can be
used in either commercial or residential markets.

According
to a recent study performed by Ducker Research Co. (May 2001--Study of the 2000
North American Fabricated Glass Market), the market for coated products has
grown significantly over the past five years. This is due to the strong demand
in construction markets and the increased demand for high-performance glazing,
particularly low-E glass. Reflective glass shipments have also grown, due
primarily to the increase in overall glazing demand. However, forecasts predict
declining shipments over the next few years, based on a softening market (which
we are now experiencing), and a decline in market penetration as
multi-functional low-E glasses increase their share. We have also seen a
dramatic increase in the use of silk-screened ceramic frit, combined with low-E
coatings and the increased use of laminated glass. The trend in glass selection
is changing quickly from approximately 70 percent reflective and 30 percent
low-E, to 70 percent low-E and 30 percent reflective. This is due to the efforts
on the part of architects to achieve more natural light in their building
designs with less “mirror-like” appearance.

Customers
in different areas of the world are ordering glass systems featuring different
color combinations, reflectance levels, solar heat gain coefficients and other
performance characteristics.

Thanks
to new coating technologies and recently developed manufacturing processes, flat
glass manufacturers are well-equipped to meet specialized demands. To satisfy
customers’ energy-control requirements, a comprehensive line of low-E glass
has been developed, tailored to different geographic regions and seasonal
heating/cooling requirements. For example, AFGD Glass’ Comfort ES 72N is a
solar control type low- E coating, with an excellent U-value and good solar heat
gain control. Other types of low-E coatings may be designed for cold climates,
still with excellent U-value, but will allow more heat from the sun to enter the
house or building for more effective passive solar heating.

Performance
Standards

Performance
standards change from year to year and from region to region.Various states and provinces have passed legislation geared to reducing
energy consumption and controlling costs for both residential and commercial
markets.Requirements
are driven mostly by local building codes and regulations. Texas recently
adopted the energy efficient chapter of the International Residential Code (IRC),
as it existed on May 1, 2001, for single-family residential construction. It
adopted the International Energy Conservation Code (IECC) for all other
residential, commercial and industrial construction. According to Senate Bill 5
(SB5), all counties in the State of Texas are included except for 16 panhandle
counties that do not meet the 3,500 Heating Degree Days requirement. Under SB5,
windows must have a solar heat gain coefficient of 0.40 or lower, or a shading
coefficient of 0.48 or lower.

Internationally,
the city of Shanghai limits the use of glass to 40 percent of the exterior
building facade for improved energy efficiency. The People’s Republic of China
has mandated the use of tempered glass in its buildings. Currently in Singapore,
the exterior reflectance of the glass must meet the maximum 10 percent.Heat soaking of tempered glass is now mandatory in Hong Kong and in some
parts of China and is being specified more and more--not only overseas, but also
in North America.

A
building’s annual energy requirements, the impact of HVAC and interior
lighting also have a bearing on performance requirements.

Architects
worldwide have increased their focus on controlling energy costs for both
heating and air conditioning. With the continued growth of low-E coatings in
building envelopes, architects continue to look for new combinations of
reflective and low-E coatings to comply with energy cost goals for specific
individual buildings. In hot climates, such as Australia, the primary concern is
minimizing solar heat gain. However, in Canada, and much of the United States,
solar heat is an important source of “free energy” during the cold winter
months, particularly for residential housing. Canada’s climate has dictated an
increasing popularity of high light and solar energy transmitting glass, as well
as growth of argon filling and triple glazing to minimize the use of both
heating and cooling, and maximize year-round energy efficiencies.

Custom
Specifications

Whatever
the reasons--aesthetics, energy control or environmental factors, architects
around the world are no longer satisfied with a “one product fits all”
approach when they specify glass products. Today, more than ever, manufacturing
facilities are receiving a wide variety of custom specifications.

As
with any other product, high-performance glass is generally called for using one
of three specifying methods:

Descriptive.
Detailed description of the physical properties, quality, and workmanship
required for the proper installation of a product of material. (i.e., a
solar-control type low-E coating that combines very high light transmittance
with sufficient solar heat gain control, while maintaining a neutral
appearance similar to uncoated glass).

Customers
today demand a high-quality product. To ensure that glass products meet certain
minimum quality and performance standards, there are several certification
agencies in place: IGMA and IGCC for sealed units and SGCC for tempered glass
products. In addition, there are a number of national standards for most types
of glass products, such as CAN/CGSB in Canada and ASTM in the United States.Associations such as IGMA, SIGMA, GANA and AAMA, also publish very useful
information pertaining to glass products and their uses.

High-performance
glass also requires unique fabrication capabilities and quality control. Once on
site, extreme care must be exercised when glazing and cleaning the glass to
ensure that it is not damaged. Reflective coatings that are not insulated,
laminated or otherwise protected can be scratched. Once glazed, consideration
should be given to protecting the coated surface until construction and all
sub-trade work is completed.

The
technology available today meets the unique specifications of architects around
the world: combining low-E glass with a full offering of high performance/medium
performance coatings, different color options, insulated glass configurations,
and special processes such as tempering, heat strengthening and laminating, to
name a few. Clearly, there has been great progress in improving window energy
efficiency in the last decade, but even greater evolution lies ahead. Many more
new innovations, such as self-cleaning glass and “smart windows,” which can
be programmed to modulate the flow of light or solar heat across the window
automatically, are being developed. High-performance glass products will
continue to be the right choice for architects and designers worldwide.